Not applicable.
Not applicable.
The present invention relates to an improved method for producing large diameter, premium quality ingots of nickel base superalloys. The present invention more particularly relates to a method for producing ingots of nickel base superalloys, including Alloy 718 (UNS N07718) and other nickel base superalloys experiencing significant segregation during casting, and wherein the ingots have a diameter greater than 30 inches (762 mm) and are substantially free of negative segregation, are free of freckles, and are free of other positive segregation. The present invention also is directed to ingots of Alloy 718 having diameters greater than 30 inches (762 mm), as well as to any ingots, regardless of diameter, formed using the method of the invention. The method of the present invention may be applied in, for example, the manufacture of large diameter, premium quality ingots of nickel base superalloys that are fabricated into rotating parts for power generation. Such parts include, for example, wheels and spacers for land-based turbines and rotating components for aeronautical turbines.
In certain critical applications, components must be manufactured from nickel base superalloys in the form of large diameter ingots that lack significant segregation. Such ingots must be substantially free of positive and negative segregation, and should be completely free of the manifestation of positive segregation known as xe2x80x9cfrecklesxe2x80x9d. Freckles are the most common manifestation of positive segregation and are dark etching regions enriched in solute elements. Freckles result from the flow of solute-rich interdendritic liquid in the mushy zone of the ingot during solidification. Freckles in Alloy 718, for example, are enriched in niobium compared to the matrix, have a high density of carbides, and usually contain Laves phase. xe2x80x9cWhite spotsxe2x80x9d are the major type of negative segregation. These light etching regions, which are depleted in hardener solute elements, such as niobium, typically are classified into dendritic, discrete, and solidification white spots. While there can be some tolerance for dendritic and solidification white spots, discrete white spots are of major concern because they frequently are associated with a cluster of oxides and nitrides that can act as a crack initiator.
Ingots substantially lacking positive and negative segregation and that are also free of freckles are referred to herein as xe2x80x9cpremium qualityxe2x80x9d ingots. Premium quality nickel base superalloy ingots are required in certain critical applications including, for example, rotating components in aeronautical or land-based power generation turbines and in other applications in which segregation-related metallurgical defects may result in catastrophic failure of the component. As used herein, an ingot xe2x80x9csubstantially lacksxe2x80x9d positive and negative segregation when such types of segregation are wholly absent or are present only to an extent that does not make the ingot unsuitable for use in critical applications, such as use for fabrication into rotating components for aeronautical and land-based turbine applications.
Nickel base superalloys subject to significant positive and negative segregation during casting include, for example, Alloy 718 and Alloy 706. In order to minimize segregation when casting these alloys for use in critical applications, and also to better ensure that the cast alloy is free of deleterious non-metallic inclusions, the molten metallic material is appropriately refined before being finally cast. Alloy 718, as well as certain other segregation-prone nickel base superalloys such as Alloy 706 (UNS N09706), are typically refined by a xe2x80x9ctriple meltxe2x80x9d technique which combines, sequentially, vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). Premium quality ingots of these segregation-prone materials, however, are difficult to produce in large diameters by VAR melting, the last step in the triple melt sequence. In some cases, large diameter ingots are fabricated into single components, so areas of unacceptable segregation in VAR-cast ingots cannot be selectively removed prior to component fabrication. Consequently, the entire ingot or a portion of the ingot may need to be scrapped.
VAR ingots of Alloy 718, Alloy 706, and other nickel base superalloys such as Alloy 600, Alloy 625, Alloy 720, and Waspaloy, are increasingly required in larger weights, and correspondingly larger diameters, for emerging applications. Such applications include, for example, rotating components for larger land-based and aeronautical turbines under development. Larger ingots are needed not only to achieve the final component weight economically, but also to facilitate sufficient thermomechanical working to adequately break down the ingot structure and achieve all of the final mechanical and structural requirements.
The melting of large superalloy ingots accentuates a number of basic metallurgical and processing related issues. Heat extraction during melting becomes more difficult with increasing ingot diameter, resulting in longer solidification times and deeper molten pools. This increases the tendency towards positive and negative segregation. Larger ingots and electrodes can also generate higher thermal stresses during heating and cooling. While ingots of the size contemplated by this invention have been successfully produced in several nickel base alloys (for example, Alloys 600, 625, 706, and Waspaloy) Alloy 718 is particularly prone to these problems. To allow for the production of large diameter VAR ingots of acceptable metallurgical quality from Alloy 718 and certain other segregation-prone nickel base superalloys, specialized melting and heat treatment sequences have been developed. Despite these efforts, the largest commercially available premium quality VAR ingots of Alloy 718, for example, are currently 20 inches (508 mm) in diameter, with limited material produced at up to 28-inch (711 mm) diameters. Attempts at casting larger diameter VAR ingots of Alloy 718 material have been unsuccessful due the occurrence of thermal cracking and undesirable segregation. Due to length restrictions, 28-inch VAR ingots of Alloy 718 weigh no more than about 21,500 lbs (9772 kg). Thus, Alloy 718 VAR ingots in the largest commercially available diameters fall far short of the weights needed in emerging applications requiring premium quality nickel base superalloy material.
Accordingly, there is a need for an improved method of producing premium quality, large diameter VAR ingots of Alloy 718. There also is a need for an improved method of producing ingots of other segregation-prone nickel base superalloys that are substantially free of negative segregation, are free of freckles, and substantially lack other positive segregation.
In order to address the above-described needs, the present invention provides a novel method of producing a nickel base superalloy. The method may be used to cast VAR ingots of premium quality from Alloy 718 in diameters greater than 30 inches (762 mm) and having weights in excess of 21,500 lbs (9772 kg). It is believed that the method of the present invention also may be applied in the production of large diameter VAR ingots from other nickel base superalloys subject to significant segregation during casting, such as, for example, Alloy 706.
The method of the present invention includes the initial step of casting a nickel base superalloy within a casting mold. This may be accomplished by VIM, argon oxygen decarburization (AOD), vacuum oxygen decarburization (VOD), or any other suitable primary melting and casting technique. The cast ingot is subsequently annealed and overaged by heating the alloy at a furnace temperature of at least 1200xc2x0 F. (649xc2x0 C.) for at least 10 hours. (As used herein, xe2x80x9csubsequentxe2x80x9d and xe2x80x9csubsequentlyxe2x80x9d refer to method steps or events that occur immediately one after another, but also refer to method steps or other events that are separated in time and/or by intervening method steps or other events.) In a subsequent step, the ingot is applied as an ESR electrode and is electroslag remelted at a melt rate of at least 8 lbs/min. (3.63 kg/min.). The ESR ingot is transferred to a heating furnace within 4 hours of complete solidification, and is subsequently subjected to a post-ESR heat treatment. The heat treatment includes the steps of holding the alloy at a first furnace temperature of 600xc2x0 F. (316xc2x0 C.) to 1800xc2x0 F. (982xc2x0 C.) for at least 10 hours, and then increasing the furnace temperature, in either a single stage or in multiple stages, from the first furnace temperature to a second furnace temperature of at least 2125xc2x0 F. (1163xc2x0 C.) in a manner that inhibits thermal stresses within the ingot. The ingot is held at the second temperature for at least 10 hours to provide the ingot with a homogenized structure and with minimal Laves phase.
In some instances, the ESR ingot may be cast with a diameter that is larger than the desired diameter of the VAR electrode to be used in a subsequent step of the method. Therefore, the method of the present invention may include, subsequent to holding the ESR ingot at the second furnace temperature, and prior vacuum arc remelting, mechanically working the ESR ingot at elevated temperature to alter dimensions of the ingot and to provide a VAR electrode of the desired diameter. Thus, after the ESR ingot has been held at the second furnace temperature, it may be further processed in one of several ways, including cooling to a suitable mechanical working temperature or cooling to about room temperature and subsequently reheating to a suitable mechanical working temperature. Alternatively, if adjustment of ingot diameter is unnecessary, the ingot may be directly cooled to room temperature and subsequently processed by vacuum arc remelting without the step of mechanical working. All steps of cooling and reheating the ESR ingot subsequent to holding the ESR ingot at the second temperature are carried out in a manner that inhibits thermal stresses and that will not result in thermal cracking of the ingot.
In a subsequent step of the present method, the ESR ingot is vacuum arc remelted at a melt rate of 8 to 11 lbs/minute (3.63 to 5 kg/minute) to provide a VAR ingot. The VAR melt rate is preferably 9 to 10.25 lbs/minute (4.09 to 4.66 kg/min), and is more preferably 9.25 to 10.2 lbs/minute (4.20 to 4.63 kg/minute). The VAR ingot preferably has a diameter greater than 30 inches (762 mm), and more preferably has a diameter of at least 36 inches (914 mm).
The present invention is further directed to a method of producing a nickel base superalloy that is substantially free of positive and negative segregation and that includes the step of casting in a casting mold an alloy selected from Alloy 718 and other nickel base superalloys subject to significant segregation during casting. The cast ingot is subsequently annealed and overaged by heating at a furnace temperature of at least 1550xc2x0 F. (843xc2x0 C.) for at least 10 hours. The annealed ingot is subsequently electroslag remelted at a melt rate of at least about 10 lbs/min. (4.54 kg/min.), and the ESR ingot is then transferred to a heating furnace within 4 hours of complete solidification. In subsequent steps, the ESR ingot is subjected to a multi-stage post-ESR heat treatment by holding the ingot at a first furnace temperature of 900xc2x0 F. (482xc2x0 C.) to 1800xc2x0 F. (982xc2x0 C.) for at least 10 hours. The furnace temperature is subsequently increased by no more than 100xc2x0 F./hour (55.6xc2x0 C./hour) to an intermediate furnace temperature, and is subsequently further increased by no more than 200xc2x0 F./hour (111xc2x0 C./hour) to a second furnace temperature of at least 2125xc2x0 F. (1163xc2x0 C.). The ingot is held at the second furnace temperature for at least 10 hours. The ESR ingot may be converted to a VAR electrode of appropriate dimensions, if necessary, and is subsequently vacuum arc remelted at a melt rate of 8 to 11 lbs/minute (3.63 to 5 kg/minute) to provide a VAR ingot. If desired, the VAR ingot may be further processed, such as by a homogenization and/or suitable mechanical conversion to desired dimensions.
The present invention also is directed to VAR ingots produced according to the method of the invention. In addition, the present invention is directed to VAR ingots of Alloy 718 which have a diameter greater than 30 inches, and is further directed to premium quality Alloy 718 ingots having a diameter greater than 30 inches and which are produced by VAR or by any other melting and casting technique.
The present invention also encompasses articles of manufacture produced by fabricating the articles from ingots within the present invention. Representative articles of manufacture that may be fabricated from the ingots of the present invention include, for example, wheels and spacers for use in land-based turbines and rotating components for use in aeronautical turbines.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend such additional advantages and details of the present invention upon carrying out or using the invention.